1. Field of the Invention
The present invention relates to an image forming apparatus that employs an electrophotographic system and, more particularly to, an image forming apparatus of a copying machine, a printer, a facsimile machine, or the like.
2. Description of the Related Art
A variety of color image forming apparatuses that utilize an electrophotographic recording system to form color images on transfer materials have been devised, and some of them have become commercially practical.
One of the aforesaid typical image forming apparatuses includes a rotary developing unit equipped with developing devices for four colors, e.g., yellow, magenta, cyan, and black, that contain toners as the developers of these colors and are disposed in a developing order in relation to photosensitive members serving as image bearing members. The electrostatic latent images of the different colors, which have been formed on the same single photosensitive member, are turned into visible toner images by the respective developing devices at a predetermined developing position. Each toner image is transferred onto a transfer medium or a transfer material, such as paper, borne and conveyed by a transfer belt or the like functioning as a transfer material conveying member. This series of steps is repeated to form a multicolor toner image.
There has been proposed another type of apparatus in which different color toner images are selectively superimposed in sequence on a photosensitive member thereby to form or develop a multicolor toner image on the photosensitive member, then the multicolor toner image is transferred all at once onto a transfer medium.
There is still another image forming apparatus employing an xe2x80x9cinline systemxe2x80x9d in which different color toner images are formed on a plurality of photosensitive members by developing devices for the different colors, and the different color toner images on the photosensitive members are sequentially and superimposedly transferred onto a transfer material conveyed by a transfer belt or the like working as a transfer material conveying member, thereby producing a multicolor toner image. Furthermore, there is an intermediary transfer type image forming apparatus. In this type of apparatus, a toner image is not directly transferred onto a transfer material from each photosensitive member; instead, the different color toner images are sequentially and superimposedly transferred onto an intermediary transfer member serving as a transfer medium, then the multicolor toner image on the intermediary transfer member is transferred onto a transfer material all at once.
Each of these typical systems of the color image forming apparatuses employing the electrophotographic recording method described above has its merits and demerits. The inline system is more advantageous than others in the aspect of meeting the recent market demand for higher speed, while the intermediary transfer system is more advantageous than others in that it is capable of handling a wider variety of transfer materials, including cardboard.
Referring now to the schematic sectional view shown in FIG. 6, a conventional example of an inline type color image forming apparatus will be described.
As shown in FIG. 6, photosensitive drums 1a through 1d disposed so that they respectively oppose developing means 4a through 4d, such as developing devices, containing different color toners are arranged in a direction in which an intermediary transfer belt 6 moves. The different color toner images that have been formed on the photosensitive drums 1a through 1d by the developing means 4a through 4d undergo a first transfer process in which the toner images are electrostatically superimposed on the intermediary transfer belt 6 in succession by transferring rollers 8a through 8d to form a full-color toner image composed of toners of four colors, namely, yellow, magenta, cyan, and black. Then, in a second transfer process, the full-color toner image is transferred onto a transfer material P from the intermediary transfer belt 6 all at once, and heated and pressurized onto the transfer material P by a fixing device (not shown), thereby turning into a permanent image.
Furthermore, charging means 2a through 2d, exposing means 3a through 3d, and developing means 4a through 4d for forming different color toner images on the photosensitive drums are disposed around the photosensitive drums 1a through 1d. 
There are also provided cleaning devices 5a through 5d having cleaning blades that frictionally slides off the residual toners remaining on the photosensitive drums to collect them after transferring the different color toner images onto the intermediary transfer belt 6.
An operation for forming an image will now be described. The exposing means 3a through 3d apply laser beams that have been modulated based on image data received from a host, such as a personal computer, to the surfaces of the photosensitive drums 1a through 1d that have been uniformly charged by charging rollers, which correspond to the charging means 2a through 2d, thereby forming desired electrostatic latent images of the different colors. The latent images are inversely developed at the developing positions by the developing means 4a through 4d into visible toner images, the developing means 4a through 4d being the developing devices that contain the toners for the different colors and are disposed facing against the photosensitive drums. The toner images are then transferred in succession onto the intermediary transfer belt 6 at the transfer positions, and simultaneously transferred onto the transfer material P that is fed with predetermined timings by a feeding means (not shown) and conveyed by a conveying means. The color toner image on the transfer material P is heated and melted by the fixing device (not shown) so as to be permanently fixed on the transfer material, thus providing a desired color print image.
Hitherto, there has been proposed a method in which the peripheral speeds of the photosensitive drums and the peripheral speed of the intermediary transfer belt at the transfer positions are set different from each other in order to improve the efficiency of transfer of toner images from the photosensitive drums onto the intermediary transfer belt. This will improve the transfer efficiency of different color toner images, making it extremely advantageous for superimposedly transferring toner images of two or more colors. Especially in a full-color mode involving four colors, the provision of the speed difference between the photosensitive drums and the intermediary transfer belt enables a good image with a uniform hue to be achieved by improving the transfer efficiency by preventing a xe2x80x9chollow imagexe2x80x9d problem in which a toner image that has been transferred from a photosensitive drum to the intermediary transfer belt misses its internal image section.
The foregoing color image forming apparatus is adapted to allow selection between the full-color mode for forming full-color toner images using the toners of the four colors and a monocolor mode (monochrome mode) whereby only a photosensitive drum for a black toner is used to form a monochrome image.
The image forming apparatus, however, has been presenting the following problem in the monocolor mode.
In the monocolor mode wherein only the black toner is used to produce a monochrome image, no toner images are formed on the photosensitive drums of the colors other than the photosensitive drum on which a black toner image is formed. However, the photosensitive drums for the remaining colors continue to run with the aforesaid speed difference maintained in the four-full-color mode in relation to the intermediary transfer belt 6. This means that the photosensitive drums of the remaining colors wastefully slide against the cleaning blades and the intermediary transfer belt 6, leading to wear or scratches. As a result, there have been some cases where the photosensitive drums reach their service life even if the toners contained in the developing devices have not yet run out or even if images have not yet been formed on a predetermined number of transfer materials.
A possible solution to the above problem is, for example, to space the intermediary transfer belt 6 apart from the photosensitive drums for the remaining colors, as necessary, in the monocolor mode. This, however, would give rise to a problem in that a separating device for spacing the intermediary transfer belt 6 away from the photosensitive drums of the remaining colors is required, resulting in extremely high cost or an increased size of the apparatus.
Accordingly, it is an object of the present invention to provide an image forming apparatus capable of preventing the service life of an image bearing member from expiring sooner than expected, without requiring an increase in size, complication in design, or increased cost of the apparatus.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings).